This invention relates generally to controlling the temperature stability of an inductor, and more particularly to methods and apparatus for controlling the temperature stability of a sensing coil of a proximity probe.
At least some known rotating and reciprocating machinery use an eddy current or proximity probe to facilitate monitoring machine vibration or rotor position characteristics. The environment that the proximity probe operates may be relatively harsh.
Typically, the proximity probe, outputs a signal correlative to a spacing between a target object such as, a rotating shaft of a machine or an outer ring of a rolling element bearing and a sensing coil of the proximity probe. The gap or spacing between the target and the sensing coil of the proximity probe needs to remain within the linear range of the proximity probe for providing accurate and reliable measurements of machine vibration characteristics. Accordingly, to provide accurate and reliable measurement, a proximity probe should remain in the linear range of operation under all operating environmental conditions.
The electronics associated with the proximity probe typically incorporates an oscillation circuit whose amplitude of oscillations is dependent on the conductance of the sensing coil. When the circuit is oscillating, the sensing coil has an alternating current flowing therein which causes the sensing coil to radiate energy in the form of an alternating magnetic field. The target object absorbs some of the radiated energy from the sensing coil when it is placed within the alternating field emanating from the sensing coil. This absorption of energy is a result of the alternating field generating eddy currents in the object which circulate so as to oppose the alternating field which created them. The amount of energy absorbed by the target object is correlative to the spacing between the target object and the sensing coil. The closer the target is to the sensing coil, the more energy the target will absorb from the sensing coil as a result of the eddy current principle. Therefore, the amplitude of oscillations of the oscillation circuit will vary as a function of spacing between the sensing coil and the target.
The ability to provide accurate and reliable measurements over a wide range of circuit and environmental conditions is dependent, at least partially on, the characteristics of the sensing coil including the material and diameter of the wire used to wind the coil, the operating frequency of the system and the electronics of the proximity probe system. The sensing coil and the remaining electronics usually have a wide range of tolerances, such as gain, bias voltage, bias current and temperature coefficients. Accordingly, each production unit has to be initially calibrated to incorporate those tolerances. Moreover, the sensing coil of the proximity probe usually contains sources of temperature drift error which are attempted to be compensated for in the final product.
A source of the temperature drift error in the sensing coil is due to a temperature dependent resistance of the coil and an inductance of the coil. This temperature dependent resistance and inductance of the sensing coil effects a source of temperature drift error resulting in inaccurate proximity probe measurements as a consequence of the false appearance of a gap change between the target and sensing coil. Such inconsistencies in temperature stability of the proximity probe result in unpredictable and unreliable measurements even when the proximity probe is functioning in its linear range of operation. Compensation for such inconsistencies is usually only partially effective to facilitate temperature dependent errors.